Any evaluation of a drive mechanism, whether hydraulic, pneumatic or electric, must first consider whether the drive is functionally acceptable for its intended use. Once that fundamental consideration has been addressed, other characteristics of the drive train, such as the purchase rice, the performance/reliability/maintenance costs and the operating (energy) costs must be evaluated.
An injection molding machine performs a molding cycle which includes the general steps or phases of clamp, inject, recover and eject. A general purpose injection molding machine must have the flexibility to perform the molding cycle for a wide variety of plastic materials, and more importantly, for a wide variety of molding applications. Limitations in the drive trains are oftentimes addressed, today, by complicated mold designs and extensive runner systems. For example if the machine can not deliver a long travel stroke at very fast speeds or high acceleration rates, an expensive multi-branch runner system for a mold with multiple feeders and gates is typically designed so that the injection stroke can be shortened and the injection speed slowed. In this manner an inferior drive train can be made to "work" and, most times, the machine end user is not aware of the limitations, even in competitive bidding situations.
Further complicating an evaluation of the drive trains are the tremendous improvements recently made in the control art which are able to mask or compensate to some extent for inherent weaknesses in a drive train. For example some control techniques developed in the precise, relatively low power/low torque machine tool and robotic industries have been literally copied into heavy duty injection molding machines exerting forces measured in tonnages. On the other hand, because improved control techniques are available for all drive trains and since the control is an inherent part of the drive train, itself, it simply must be evaluated along with and as part of the drive train.
Injection molding machines have traditionally been operated with hydraulic systems as their primary source of motive power. The most important step in the molding cycle, the injection step, is inherently suited to a fluid drive train system. A fluid drive train system using fluid pressure hydraulicly coupled to a prime mover has an innate ability to directly correlate to the movement of the injection molding material into the mold cavity. While relatively recent developments in velocity profiling have now standardized injection ram control, the hydraulic pressure exerted on the "ram" by the hydraulic drive directly corresponds to the pressure of the melt in the mold and is important for controlling or establishing the velocity profiling. Fundamentally then, a hydraulic system provides a direct measure of what is happening in the mold, whereas other drive systems, specifically an electric drive, can only provide an indirect measurement. This distinction becomes significant when considering mold packing. A hydraulic drive can easily maintain a packing pressure through direct pressure sensing. An electric drive has to use separate transducers to measure melt pressure (importantly affected by its position in the mold) and has to switch to torque control (and the torque has to be correlated to pressure which is not necessarily linear because of slip in the drive) at slow or zero velocity where torque pulsations from the motor can adversely affect the molded part. In addition, there is a non-linear translation of forces before static friction is overcome and the mechanical coupling engages in the electric drive train. That static friction can be variable.
There are also fundamental differences between electric and hydraulic drive mechanisms in the speed and speed control during injection. Each drive train has some advantages and disadvantages in this regard. However, for reasons discussed below, a hydraulic drive can linearly move the screw faster over a longer stroke than an electric drive.
In this regard, it must be noted that electric drive injection molding machines have been present in various forms for a number of years and have recently been promoted for general purpose injection molding machine applications to which this invention relates. Screw translation in an electric drive machine is typically accomplished by a ball screw coupling and recent developments in ball screw couplings have rendered such systems acceptable for a wide variety of plastics and plastic applications. Nevertheless, electric motors employing mechanical drive couplings (such as ball screw couplings) while initially quicker, cannot provide the rapid acceleration characteristics of a fluid drive train. As the science of molding plastics continues to evolve, the velocity profile spectrum of a fluid system will remain superior to that achieved by the mechanical couplings of electric drive machines. This feature coupled with the direct pressure sensing/control concept previously discussed provides hydraulic drive systems functional advantages over electric drive machines. Again the discussion is limited to general purpose injection molding machines which must possess a wide range of operating characteristics. Certain molding applications requiring relatively slow or medium speed injection strokes are adequately handled by electric drives. This is especially true with the advances in the control technology. However, the advances in control technology also apply to hydraulic drives and the consideration again reduces to the fundamental distinctions between the drive trains.
A similar functional analysis can be likewise applied to the clamping step of the molding process. Hydraulic clamp drives function like a press in that a high pressure is exerted to generate a high tonnage force and the valve simply closed to lock the mold halves together at a high pressure while pump pressure is released. The electric drive can only function in a similar manner by being left "on" rotating at "zero" velocity with maximum torque. This produces undesirable motor effects effectively limiting the electric drive to "toggle" clamp applications where the zero velocity characteristics of the motor are less noticeable.
Apart from functional considerations, when the other decision factors mentioned above are considered, hydraulic drives have advantages and disadvantages. Hydraulic drives are relatively inexpensive. They are tried and proven drives suitable for performing all the steps of a molding cycle and they have proven themselves rugged and reliable over the years. Further, in almost all instances, hydraulic fluid arrangements are present at the molding facility, i.e., setting and pulling cores. Any injection molding machine, whether electric or hydraulic, therefore must have the ability to handle hydraulics. Since the facilities use hydraulics and the machines must interface with hydraulic arrangements to perform the molding cycle, the environment is suited to hydraulic drive trains on the machine.
There are disadvantages however. Hydraulic oil is subject to dirt and contamination and requires filtering and maintenance and, in addition, has a potential for oil leakage. In addition, the system must employ heat enchangers and coolers to maintain the oil temperature relatively constant because, as generally known, variations in oil temperature produce variations in the drive which, in turn, have to be compensated for by the control system. However, the control systems have made dramatic improvements in recent years to compensate for such variations so that all of these "disadvantages" associated with the hydraulic drive are insignificant.
The principle disadvantage of the hydraulic drive in an injection molding machine is simply that it uses more energy than an electric drive. The energy cost to drive the electric motor powering the pump(s) is higher in a hydraulic drive than that of an electric drive because the electric drive only actuates the motor at the speed and power necessary to perform the molding step required at any given time in the molding cycle. In marked contrast, the motor is always driving the pump(s) in a hydraulic drive. In fact, some slight flow should be present in a hydraulic drive to maintain system heat.
Typically the molding machine's constant velocity pump(s) directs its fluid output to various hydraulic actuators on the machine through a proportioning valve which is variably opened or closed depending on the machine demand. Alternatively, a variable volume pump is used in lieu of the proportioning valve/constant delivery pump. The motor driving the pump powers the pump at a speed and torque necessary for the pump to deliver fluid flow and pressure on demand vis-a-vis the proportioning valve (or a functionally similar arrangement on the variable flow pump). When high pressure/flow is not required, the pump simply recirculates the fluid to a sump. The motor speed is directly proportional to energy usage. Because the pump must deliver power on demand, the motor is typically rotated at a high constant speed to make sure the pump can deliver the power. Because the pump(s) are idle during portions of the molding cycle, the power is simply recirculating fluid from and to the sump thus increasing the energy operating costs of the machine.
The prior art has long recognized this problem and has made attempts to control the speed of the motor to match the pump requirements dictated by the molding cycle. In concept, this is a viable approach which not only reduces energy usage by the motor but also reduces heat generated by the fluid. This concept was realized in Collins U.S. Pat. No. 3,911,677 but addressed through a series of on-off switches to a DC motor, obviously a brush type, vis-a-vis an interface unit not acceptable in today's environment. A more recent improvement is disclosed in Jones et al. U.S. Pat. No. 4,904,913 and an add-on device functionally similar to Jones is disclosed in the February, 1986 issue of Plastics World in an article entitled "IMM Power Use Cut by 70%" (p. 15). Such systems, while advanced over Collins, still use a set value for a machine function at which the machine has to be separately programmed and at which the motor is ramped. In Holzschuh U.S. Pat. No. 5,580,585, the concept of varying motor speed between set limits is also disclosed with a motor described as a steplessly regulated motor and then further stated to be a vector controlled motor. Like Jones and Collins, set signals are simply inputted to the motor. In Wurl U.S. Pat. No. 5,093,052 the systems disclosed in Collins, Jones and Holzschuh is materially improved on by the use of feedback to control pump outputs.
Japanese Patent Publication Number 63053302, dated Jul. 3, 1988 shows integration of the machine control with the control of motor speed for driving the pump. The integration is done through analog circuits. In Hertzer et al. U.S. Pat. No. 5,052,909 the integration of the motor pump is accomplished through the machine control but in an arrangement driving a variable speed pump. In general summary, the prior art shows numerous attempts to reduce the energy used by the hydraulic drive of an injection molding machine by varying the speed of the motor driving the pump during the molding cycle. These techniques have only been partially successfully in reducing the energy operating costs of the injection molding machine to what is otherwise possible by the use of systems that are added onto the basic control system of the machine or require modifications to the basic control system, and in all instances ramp the motor to set values. That is given the response time of the system to supply high volume flow under pressure the defects in the speed variation systems disclosed in the prior art prevent the systems from being as energy efficient as otherwise possible.
As already noted, because the motor is actuated only when called upon to actuate a molding function in the molding cycle and because the power is supplied only at the amount required, an electric drive is more energy efficient than the hydraulic drives which constantly run the electric motor as described above. This inherent feature of an electric drive coupled with the advances in motor control technology has resulted in the recent introduction into the marketplace of a number of general purpose all electric injection molding machines.
Generally speaking, it has long been known to use DC brush type motors to power the drive of an injection molding machine, but such drives are not economical. First, a wound armature is needed. Second, brushes for such motors generate sparks and wear. Third, brush commutation produces low speed torque ripple or cogging.
More recently DC brushless motors have been commercialized for all electric drive injection molding machines as shown in Faig et al. U.S. Pat. No. 4,988,273. ("All electric" is actually a misnomer since a hydraulic system is almost always used for the ejection and/or core placement step of the molding cycle by "all electric" injection molding machines. As used throughout the specification, "all electric" is used in the conventional sense to describe an injection molding machine having electric motors for performing the injection, clamp and recover steps of the molding cycle.) The DC brushless motor eliminates the brushes in a ceramic magnet/wound stator arrangement with a Hall effect sensor or encoder to emulate the performance of the DC brush type motor. Unfortunately cogging and zero velocity pulsations are present due to the induced trapezoidal wave form requiring additional control techniques to minimize their adverse effects. This is discussed at some length in Faig U.S. Pat. No. 4,988,273. In addition, brushless motors are relatively expensive for large size applications typically employed in a general purpose injection molding machine.
The cogging and torque pulsations of the DC drive are inherently reduced by the sinusoidal currents resulting in an AC motor. Specifically, AC synchronous drives initially developed in the robotics field have been successfully applied for a number of years in all electric machines. The high precision demanded for machine tool and robotics application built into such drives makes them ideally suitable for precision molding machine applications. However, AC synchronous motors use rare earth magnets and are expensive. Because of their low inertia rotor, they are limited to small horsepower applications. As a result, AC synchronous motors have to be ganged for operating larger, general purpose injection molding machines. This significantly increases the cost of the drive.
AC induction motors have long been used for driving the pumps in an injection molding machine. The low cost of the "squirrel cage" motor coupled with its high torque capabilities have made such motors an obvious choice of the machine designer. The controls for AC induction motors have typically been limited to operate the motor at steady speeds, which, as noted above, fit well within the conventional pump arrangement of the injection molding machine.
With the development of vector control, the application for AC induction motor drives has been significantly expanded. This control technique is normally attributed to the work of the Siemen's inventor, Felix Blaschke in the early 1970's, who it is believed initially suggested, in one of his early patents, that the vector control drive for asynchronous motors could be used for an extruder. Despite Blaschke's early suggestion, it is only recently that vector control AC induction drives have been used in injection molding machines. Japanese Patent publication No. 1-135609 (Japanese application No. 62-295606) discloses the use of an AC vector control drive for performing the injection step by an injection molding machine using a switched delta motor connection to increase the operating ranges of the AC motor and produce a wider range of plastic products from the machine. A less sophisticated application of vector control is subsequently disclosed in Faig et al. U.S. Pat. No. 5,362,222. Faig shows a common, typical vector control/pwm inverter drive for performing all the functions of an injection molding machine, including but not limited to screw translation. The prior art has thus followed Blaschke's suggestion and has now implemented vector control for achieving screw translation in an injection molding machine despite its limitations as described above.